Multi-pad hydraulic fracturing is believed a cost-effective procedure to unlock the tight oil from low-porosity, low-permeability reservoirs. However, the inconvenience of difficult-dissolving process at surface and crosslinking of the conventional guar-based fracturing fluid systems cannot satisfy such fracking jobs because of the massive proppant loading, high flow rate and large volume of the fluids used. To address these issues, a crosslinking-free and rapid-dissolution fracturing fluid system based on synthetic hydrophobically associating polymer (HAP) "water-in-oil" emulsion was developed. The HAPs are derived from classical water-soluble polymers by incorporating small amount of long hydrophobic side chains onto the polymer backbone. When above a critical associating concentration, these polymers can automatically form a three-dimensional transient network by intermolecular association, reminiscent of cross-linked structures, offering the suspending capacity for proppants. With inverse emulsion polymerization, the obtained HAP emulsions can not only get high molecular weight, but also be rapidly dispersed and finally dissolved within 5 minutes. It was found concentrated HAP polymer emulsions can be dispersed online with surface water or even produced fluids to get final designed concentration. Laboratory rheological study shows that 1% of the as-prepared fracturing fluid can reach more than 50 mPa%s at 150 0C. Compared with guar-based fluid, the HAP fracturing fluid can be completely broken, and the viscosity, surface tension, skin damage of the residual fluid on the permeability are all smaller, while the fluid loss is comparable, proppant-carrying ability is even better. Most importantly, no further surfactant was needed to assist the flowback the fluid. Since September 2013, such associative polymer fracturing fluids were successively used in 29 wells of 3 well pads, Yan-227, Yan-22 and Bin-37 blocks in Shengli Oilfield, Sinopec, where the temperature ranges from 110 to 145 cC. Totally 60,000 m3 fluids were consumed in these fracking jobs, and 87, 9, and 45 stages were successively fractured in the horizontal sections, respectively.

The hydrophobically associating polyacrylamide (HAPAM) is an important kind of water-soluble polymer, which is widely used as a rheology modifier in many fields. However, HAPAM products prepared in a traditional method show disadvantages including poor water solubility and the need for hydrocarbon solvents and appropriate surfactants, which lead to environmental pollution and increased costs. To solve these problems, we reported a novel kind of HAPAM “water-in-water” (w/w) emulsion and its solution properties. In this work, a series of cationic hydrophobic monomers with different alkyl chain lengths were synthesized and characterized. Then, HAPAM w/w emulsions were prepared by the aqueous dispersion polymerization of acrylamide, 2-methylacryloylxyethyl trimethyl ammonium chloride and a hydrophobic monomer. All these emulsions can be stored more than 6 months, showing excellent stability. An optical microscopy observation showed that the particle morphology and the particle size of the HAPAM emulsion were more regular and bigger than the emulsion without the hydrophobic monomer. The solubility tests showed that such HAPAM w/w emulsions have excellent solubility, which took no more than 180 s to dilute and achieve a homogeneous and clear solution. The rheology measurements showed that the HAPAM association increases with a hydrophobe concentration or the length of hydrophobic alkyl chains, resulting in better shear and temperature resistances. The total reduced viscosity was 124.42 mPa·s for cw101, 69.81 mPa·s for cw6-1, 55.38 mPa·s for cw8-0.25, 48.95 mPa·s for cw12-0.25 and 28 mPa·s for cw16-0.25 when the temperature increased from 30 °C to 90 °C. The cw8-2.0 that contains a 2 mol% hydrophobe monomer has the lowest value at 19.12 mPa·s due to the best association. Based on the excellent stability, solubility and rheological properties, we believe that these HAPAM w/w emulsions could find widespread applications.

Cross‐linked polymer gels have been commonly utilized in the hydraulic fracturing process for stimulating oil and gas production; however, their weak thermal stability still impedes their more broadly use. Here we report doubly cross‐linked polymer gel consists of a high‐viscosity friction reducer (HVFR), poly‐(acrylamide‐co‐acrylic acid‐co‐2‐acrylamido‐2‐methyl‐1‐propane‐sulfonic acid), and crosslinkers zirconium ion (Zr4+) and polyethyleneimine (PEI). The mixture solution of HVFR/Zr4+/PEI can be gelled spontaneously via heating to form double cross‐linked polymer gel due to the formation of physical and chemical crosslinks. The electrostatic interactions of Zr4+ and carboxylate moieties from HVFR to form first physically cross‐linked network at low temperature; the transamidation reaction between amide groups of HVFR and primary amines of PEI to generate second chemically cross‐linked network at high temperature. More significantly, the physical and chemical crosslinks can work synergistically to improve thermal stability of double cross‐linked polymer gel as compared to single cross‐linked polymer gel counterparts, which is capable of reaching fluid service temperatures up to 190°C. The double cross‐linked polymer gel features delayed crosslinking performance, the physically cross‐linked reaction of HVFR and Zr4+ occurs at approximately 52°C, while the onset of chemical crosslink of HVFR and PEI can be tuned from 154 to 180°C. Moreover, double cross‐linked polymer gel provides a high viscosity at ultra‐high temperatures, with remarkable heat‐ and shear‐resistance as well as proppant carrying capacity in hydrofracking process. The double cross‐linked polymer gel also can be completely broken with no apparent residues. This double cross‐linked polymer gel may provide a new approach for polymer gels used in ultra‐high‐temperature reservoirs.

Oil pollution results from daily activities and a variety of industries have caused not only severe environmental problems but also wastage of valuable petrochemical resources. Separation based on superwettable materials holds promise; however, practical applications of a single type of superwettable materials were often limited due to their ability in treatment of complicated oil-water systems. Herein, a Gemini-type separator was created through the cooperation of two kinds of superwettable sand particles with opposite wettability, i. e., one is superhydrophobic whereas the other is superhydrophilic.Cooperatively by the two types of superwettable sand, consecutive separation and purification of both water and oil phases from complicated oil-water systems (e. g., water mixed with a lighter or denser oil, water emulsified in oil, oil emulsified in water, and/or a combination of them in one batch) could be achieved with high flux and superior efficiency just in one single operation unit.